Display Hardware
- 2. Cathode Ray Tubes Basic operation Refresh CRT Primary component – heated metal cathode control grid heating filament
- 3. Intensity control – voltage on control grid Focusing System force the electron beam small spot of light Otherwise, electrons repel and spread Accomplished with electric or magnetic fields Deflection Plates horizontal and vertical Spots on screen by transfer of CRT beam energy to phosphor
- 4. Kinds of phosphor based on Color Persistence : time it takes the emitted light to decay one-tenth of original intensity Resolution Aspect Ratio
- 5. Raster Scan Displays Image created as Grid of Pixels – “Picture Elements” – Scan Conversion Picture definition is stored in a frame buffer memory or refresh buffer • one storage cell per pixel
- 6. Example: assume a display of 16 x 16 pixels, each of which is either “on” or “off” The electron beam traces a path across the first row, then returns to starting side and moves across the second row, and so on The beam is turned on or off as it passes over a pixel that should be on or off Here, the beam is OFF for the entire first row, OFF during the retrace, and then ON for the two pixels that are ON
- 7. Black and White (Bilevel) systems Bit Value 0: Beam turned ON Bit Value 1: Beam turned OFF Frame buffer : Bitmap For Color and intensity variations : multiple bits per pixel Frame buffer : Pixmap Refreshing : 60 – 80 frames per second Horizontal and Vertical Retrace Interlacing : with slower refreshing rates
- 8. Random Scan Display Also called as Vector or Stroke or Calligraphic Displays Draw a picture one line at a time – each image was expressed in the form of a series of {move-to, draw-to} commands – the beam could be moved to a specific x,y location while turned off, then turned-on, and a draw-to command would move the beam (while on) to a second x,y location – the result, a line drawn from the first location to the second
- 9. Example: – move to (a, b) (c, d) – draw to (c, d) – draw to (e, f) (a, b) – draw to (g, h) (e, f) – draw to (a, b) (g, h) – move to (i, j) – draw to (k,l) (i, j) (k, l) Note: Arrows just indicate the flow Refresh Display file contains the Line drawing commands
- 10. For Line Drawing applications Higher resolution Problems with random-scan – as complexity of display grows, time to draw increases – if time between refreshes grows too long, display begins to flicker
- 11. Color CRT Color CRTs are much more complicated – Phosphors on the face are laid out in a precise geometric pattern – Uses a pattern of color phosphors on the screen: Delta electron gun arrangement In-line electron gun arrangement
- 12. Color CRTs have – Three electron guns (RGB) – A metal shadow mask to differentiate the beams
- 13. General case: – Simple color display has N x M pixels and each pixel is one of 8 = 23 colors – color depth = k bits – each pixel can be one of 2k colors – if k = 3b, then there are 2b intensity levels for Red, Green, and Blue – if k ≠ 3b, then typically a color indexing scheme is used • the value stored in the frame buffer is not used to directly control the drive intensity • frame buffer value is an index into a color palette
- 14. Direct drive case – frame buffer has 3b bits/pixel (example b = 6) 3b bits / pixel Red Green Blue pixel address 000000 000111 000000 0 000100 000100 000100 1 111111 111111 111111 2 … … … … NM pixels … … … … … … … … … … … … directly controls red, green, and blue intensity
- 15. Indexed color palette case – frame buffer has k bits/pixel (example k = 8) k bits/pixel Frame Buffer pixel address 00000000 0 3b bits / row 00000010 1 Color Palette 11111111 2 Red Green Blue … … 000000 000111 000000 000100 000100 000100 … … 111111 111111 111111 … … … … … 2k … … … … … rows … … … … … …
- 16. Display Technology: LCDs Liquid Crystal Displays (LCDs) – Used in Calculators, laptop etc – Light is passed through the liquid crystal that is aligned to either block or transmit light
- 17. Display Technology: LCDs Liquid Crystal Displays (LCDs) – Used in Calculators, laptop etc – Light is passed through the liquid crystal that is aligned to either block or transmit light
- 18. Display Technology: Plasma Plasma display panels – Similar in principle to fluorescent light tubes – Small gas-filled capsules are excited by electric field, emits UV light – UV excites phosphor – Phosphor relaxes, emits some other color